It has been pointed out that myopia in the human eye not only brings inconvenience to our everyday life but also increases risk of disorders such as retinal detachment and cataract as the myopia gets deteriorated. Especially in recent years, the prevalence of myopia has been increasing so much that the social demand for technologies of myopia progression suppression is growing.
As one of such technologies of myopia progression suppression, a myopia progression suppression technology using a contact lens is proposed based on the accommodation lag theory and the off-axis aberration theory. Myopia progression especially in infants is often caused by progression of axial myopia, and since myopia and myopic progression are considered to be triggered by hyperopic focal error wherein the image is focused at a location behind the retina to develop further by having the ocular axis extended due to retinal extension that compensates the hyperopic focal error, the effect of such technology of myopia progression suppression is anticipated. The accommodation lag theory explains that the autofocus mechanism of the human eye does not fully respond to the required accommodation thereof, but as a result of the eye's tendency to work with minimum accommodation, it generates the accommodation lag, which is hyperopic focal error caused by the lack of accommodation in a condition of near vision that requires it, and therefore, the more work is involved in near vision, the more myopia progresses by having the ocular axis extended further, which is triggered by the hyperopic focal error. Also, the off-axis aberration theory deals with myopia progression prompted by the extended ocular axis triggered by generation of hyperopic focal error around the retina even with the foci coinciding on the retina in its central region because of the tendency for the myopic eyeball to take an elongated shape in the direction of ocular axis.
A specific method of suppressing myopia progression based on the accommodation lag theory and the off-axis aberration theory, as described in Japanese Domestic Publication of International Patent Application No. JP-A-2007-511803 (Patent Document 1) and International Publication No. WO96/16621 (Patent Document 2) for example, is to use a contact lens for correcting hyperopic focal error to let the light incident on the pupil focused on or before the retina.
That is, the invention described in Patent Document 1 referred to above takes a closer look at the phenomenon wherein the image focuses behind the retina in the retinal peripheral region when a monofocal contact lens is applied to a myopic eye with its optical axis grown and the lens power is set such that the central light rays focus on the retina. Therefore, as the lens for myopia progression suppression described in Patent Document 1, the one with different lens powers set at the central and peripheral portions is adopted, for example, so that one lens power is set to focus on the retina in the central region whereas the other lens power is set to focus on or before the retina in the retinal peripheral region.
However, repeated studies on contact lenses for myopia progression suppression with such conventional structure performed by the inventor found that high additional power of as much as +2.0 D is required for correcting the hyperopic focal error in the retinal peripheral region in order to obtain a desired effect of myopic progression suppression. That revealed the problems of reduced rate of light collection on the retina in far vision as well as generation of myopic focal error, leading to deterioration of subjective QOV (Quality of Vision). Especially, since the contact lens for myopia progression suppression is often worn by infants for a long period of time, the conventionally structured contact lenses for myopia progression suppression that fail to attain QOV equivalent to that of a regular contact lens for refractive correction posing a risk of disrupting the daily life of the wearer had some room for improvements.
Also, the invention described in Patent Document 2 referred to above corrects the hyperopic focal error by means of setting a value of lens power under a condition of spherical aberration wherein the focus gradually moves out toward the positive side of diopter toward the outer peripheral side in the radial direction of the lens, in addition to setting a focus that properly corrects the myopia on the optical axis, and then setting a focus on the optical axis at a location closer to the cornea. However, the invention inevitably had the same problem of significant reduction of QOV as the one described in Patent Document 1 due to the lack of clear focus as in the case of a progressive multifocal lens. Especially, there was a risk of causing disruption in daily life because a high additional power of as much as +2.0 D compared to a proper condition for myopic correction is set in anticipation for suppressing myopia progression, as is the case with the invention described in Patent Document 1.